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FET Accompli

Paul Whytock | Nov 05, 2007

Congratulations go out to Toshiba for scoring a real technological first with its development of a gallium-nitride (GaN) power field-effect transistor (FET), designed for the Ku-band (12 to 18GHz) frequency range. So, what’s special about this transistor?

Well, it achieves an output power of 65.4W at 14.5GHz, which is the highest level of performance yet reported at this frequency band. Full-scale production of the transistor is expected to start in the first quarter of 2008.

Generally speaking, technological advances in Ku-band microwave amplifiers focus on replacing the electron tubes with semiconductors and, in particular, GaN devices. What Toshiba did with this particular transistor is implement a structure that’s a high-electron mobility transistor optimised for Ku-band application. Via hole technology played its part in this development, ousting source wire bonding. This will cut parasitic inductance and enhance circuit design for work at Ku-band frequencies.

Toshiba’s development has good timing. Industry trends within the satellite microwave communications sector indicate that a steadily increasing demand for GaN power FETs for both new equipment and the replacement of electron tubes.

The reality is that rapidly increasing communications demands in satellite comms systems pushes forward the need for higher output power in signal amplifying components. The dilemma here, though, is that increased performance is always coupled with challenges relative to heat dissipation. This is an area where GaN devices provide a practical advantage over more established technologies like gallium arsenide.

Toshiba managed to create the improved performance characteristics of this new FET by optimising the composition and thickness of the AlGaN and GaN layers formed on the highly heat-conductive silicon-carbide (SiC) substrate of the HEMT structure. In addition, the company applied a shorter gate length of below 0.3 microns and optimised the shape of each electrode and element configuration to enhance heat dissipation and, therefore, boost performance within Ku-band frequencies.

To reduce the parasitic inductance and improve higher frequency performance, Toshiba also developed a special way of forming via holes in SiC substrate—a particularly demanding process.

Shortening gate lengths means that suppressing current leakage at the gate electrode is critical. In this case, the company uses an overcoat process applied around each gate electrode. It ultimately contributes to suppressing gate leakage to 1/30 that of Toshiba’s conventional approaches. An electron beam exposure technology is applied to secure stable processing of gate lengths below 0.3MICROm.

Not content with all of this, the company is now preparing to develop GaN technology for microwave frequency applications in 18 to 30GHz frequencies and beyond.